A vet visit is recommended of course. You may have to force-feed your kitten. Using an eye dropper or syringe is recommended. Feed small amounts and don't forget water. Sick kittens can pass very quickly and should be cared for when you first notice a problem.
It is extremely important that the kittens are drinking and are getting nutrients into their bodies. They should be seen by a vet to rule out diseases like distemper and to make sure they don't have parasites.